IPM for vegetables: a hot topic

March 16, 2009

Many newspaper headlines during the recent passing of the European Union’s new ‘pesticides package’ focused on the potentially serious consequences for Europe’s vegetable producers, making ENDURE’s field vegetable case study timely as well as important.

It is not difficult to understand the fears of vegetable growers. Though their produce forms an essential - and healthy - part of our diets, in the eyes of the agro-chemical companies they remain a sideline rather than a core part of their business. If certain pesticides are no longer available, growers fear there will be little interest amongst the chemical giants to produce new ones.

It all comes down to simple economics. In France, for example, some 17,500 hectares were devoted to carrots and turnips in 2007 (United Nations Food and Agriculture Organisation statistics) producing a crop of 710,000 tonnes. For wheat, the figures were 5.3m hectares and 33.2m tonnes respectively. If you were in charge of an agro-chemical company’s research and development budget, where would you be investing?

Against this background, ENDURE has a team devoted to a field vegetable case study. Under the spotlight are five major field vegetable crops: carrot, cabbage, onion, leek and salad; and within its package of work are investigations into interesting non-chemical approaches including landscape management and methods of dealing with soil-borne diseases and pests.

The first task of the team, which comprises members from nine ENDURE partners, has been to compile a comprehensive dossier of pesticide-based and alternative methods to control pests and weeds of the five major crops under investigation. They expect to complete this task by summer 2009.

Trials of various bio-fumigation crops are underway, focusing mainly on members of the brassica family. Copyright: Gerard Korthals, Wageningen UR, Netherlands.

They are also studying past and ongoing research on the control of weeds and soil-borne diseases through non-chemical means and, in particular, will examine soil steaming, soil solarisation and/or bio-fumigation, with field experiments running through this summer in three different countries (for more details on these techniques, see below).

By this time next year, the group is also scheduled to complete its report on strategies for landscape management as a means of naturally regulating pests in field vegetable crops. This involves techniques such as planting flower strips to encourage natural predators of pests, the use of trap crops to attract pests, and push-pull strategies which use a combination of trap crops to attract pests and other crops to repel them (a technique successfully used in Kenya, for example, to protect maize crops).

“Vegetable growers are increasingly considering integrated pest management (IPM) as the only way to tackle the growing lack of pesticides for many uses, and as a challenging opportunity to meet consumer demands for safer products,” said Philippe Lucas, the leader of field vegetable case study team. “This opens up avenues for research and development in this area.”

Alternatives to chemicals

The field vegetable case study is taking a closer look at three techniques for dealing with soil-borne diseases and pests that show some promise. Soil steaming
has been used for more than a century to kill soil-borne diseases in glasshouses and has proved useful at controlling weeds in field crops.

Its description describes exactly what is involved: steam is passed through the soil, heating it up and thus killing, for example, weed seeds. It was used through much of the last century in the Channel Island of Guernsey, for example, to control diseases in the glasshouses used to grow the famous ‘Guernsey Tom’ tomatoes. Giant coal-fired boilers were used and between October and Christmas each year, the island is said to have been covered in the sweet, nutty aroma of ‘cooking’ soil.

A clear-cut demonstration of the efficacy of soil solarisation on a kibbutz in Israel. Egyptian broomrape (Orobanche aegyptiaca
) has destroyed carrots in the control plots (right) but has been completely controlled in treated plots (left). Copyright: Jaacov Katan, Hebrew University of Jerusalem, Bugwood.org.

Modern machinery has made the technique possible in open fields. Typically, steam is applied under pressure beneath metal pans, raising soil temperatures to more than 70°C and killing most weed seeds up to a depth of 10cm or so. While avoiding the use of herbicides to kill weeds, soil steaming is not as environmentally friendly as one may think as it consumes lots of fuel and water.

Current experiments at the University of Aarhus, an ENDURE partner in Denmark, are focused on band-steaming, which targets the rows in which the crop will be grown, combined with manual weeding of the areas between rows. This cuts drastically the amount of fuel used, though work is still continuing on analysing the effects of steaming on soil life and its effectiveness against soil-borne diseases.

Soil solarisation
uses the power of the sun to destroy soil-borne pathogens and pests. Typically, transparent plastic sheeting is laid over a flat, even bed and left in place for four to six weeks. Used with some success in California, USA, and Israel, for example, where soil temperatures can reach 60°C at a depth of five centimetres, it is a tactic that relies on regular sunshine and also has some environmental consequences as lots of plastic is required and must be disposed off after use.

Bio-fumigation
is an interesting technique that offers opportunities to avoid using soil fumigants such as the ozone-depleting methyl bromide. Bio-fumigation typically uses plants from the brassica family (mustard plants, for example) which contain high levels of glucosinolates, compounds from which plants can derive naturally occurring pesticides.

When plants are broken up and dug into the soil (as a green manure) enzymes are released which help to create a natural fumigating gas. Studies have shown that bio-fumigation can increase crop yields, though the extent to which this is due to the destruction of soil pathogens and the part played by improved soil health thanks to the green manure has been questioned.